A major objective of this application is to obtain a better understanding of the interactions between DNA and proteins and of related biological activities including the regulation of gene expression and the mechanisms of DNA replication, transcription and recombination. Toward this end crystallographic, biochemical and mutagenic studies will be pursued of a number of DNA- and RNA-interacting proteins including the exonuclease from phage lambda, the Skn-1 developmental regulatory protein from C. elegans, the "N" antitermination protein and DNA replication proteins from phage T4 as well as various engineered and other variants of lambda Cro protein. Studies will also be continued of E. coli beta-galactosidase. The way in which proteins interact with DNA is fundamental to the correct regulation and growth of every living cell. Many human diseases have their origins in a breakdown in these basic control processes. Studies of the sort described here provide fundamental insights into the way in which proteins recognize their target sites on DNA and exert their control functions. There is every reason to expect that understandings derived from such systems in simple organisms will be relevant to human health. Knowledge of the three-dimensional structure of E. coli beta-galactosidase is expected to lead to improvements in a number of the assays that are based on the unique properties of the enzyme.